The present invention relates to error correction in data communications, and more particularly, to forward error correction (FEC). Even more particularly, the present invention relates to the selection and use of optimal Turbo Codes in high performance data communication systems, such as emerging third generation terrestrial cellular mobile radio and satellite telephone systems, for which flexibility in supporting a wide range of system requirements with respect to transmission data rates, channel coding rates, quality of service measures (e.g., latency, bit-error rate, frame error rate), and implementation complexity are highly desirable.
Forward error correction (FEC) is required in terrestrial and satellite radio systems to provide high quality communication over the RF propagation channel, which induces signal waveform and spectrum distortions, including signal attenuation (freespace propagation loss) and multi-path induced fading. These impairments drive the design of the radio transmission and receiver equipment, the design objective which is to select modulation formats, error control schemes, demodulation and decoding techniques and hardware components that together provide an efficient balance between system performance and implementation complexity. Differences in propagation channel characteristics, such as between terrestrial and satellite communication channels, naturally result in significantly different system designs. Likewise, existing communication systems continue to evolve in order to satisfy increased system requirements for new higher rate or higher fidelity communication services.
In the case of terrestrial cellular mobile radio telephony, Analog Mobile Phone System (AMPS) is an exemplary first generation system; the U.S. IS-136 and European GSM time-division multiple-access (TDMA) standards and the U.S. IS-95 code-division multiple-access (CDMA) standard are second generation systems; and the wideband CDMA standards currently under development (e.g., CDMA 2000 in the U.S. and UTRA in Europe) are third generation systems.
In the third generation systems the development of flexible, high-speed data communication services is of particular interest. Desirable features include the ability to perform rate adaptation and to satisfy a multiplicity of quality-of-service (QoS) requirements.
Traditional forward error correction (FEC) schemes for communication systems include use of convolutional codes, block codes such as Reed-Solomon or BCH codes, and/or concatenated coding schemes.
Turbo Codes are a relatively new class of block codes that have been demonstrated to yield bit error rate (BER) performance close to theoretical limits on important classes of idealized channels by means of an iterative soft-decision decoding method.
A Turbo encoder consists of a parallel concatenation of typically two systematic, recursive convolutional codes (xe2x80x9cconstituent codesxe2x80x9d) separated by an interleaver that randomizes the order of presentation of information bits to a second constituent encoder with respect to a first constituent encoder. The performance of a Turbo Code depends on the choice of constituent codes, interleaver, information block size (which generally increases with higher data rates), and number of decoder iterations. For a particular Turbo Code, in which the constituent codes are fixed, one can ideally adjust the block size and number of decoder iterations to trade-off performance, latency, and implementation complexity requirements. As the block size changes, however, a new interleaver matched to that block size is required.
In a CDMA network with synchronized base stations, forward link channels (from base station to user terminal) can be designed to be orthogonal, using, for example, Walsh-Hadamard spreading sequences. This is generally not possible, however, for reverse link channels (from user terminal to base station), which therefore operate asynchronously using spreading sequences that are only quasi-orthogonal. Thus, the reverse links in a synchronous CDMA network typically experience more interference and therefore may require stronger FEC (via lower rate codes) than the forward link channels do.
In an asynchronous CDMA network, the forward and reverse link channels are more similar in terms of interference levels, so it is possible to use a common FEC scheme (or at least more similar FEC schemes) on the two links.
The flexibility and high performance of Turbo Codes make them a potentially attractive technology for sophisticated data communications services. It is therefore desirable to identify Turbo Codes and Turbo coding FEC schemes that best match diverse service requirements with respect to data rates and coding rates while minimizing implementation complexity.
The present invention advantageously addresses the above and other needs by providing methods for designing and using universally optimized Turbo Codes.
The present invention advantageously addresses the needs above as well as other needs by providing an approach for designing universal constituent codes of Turbo Codes providing optimal performance in conjunction with a variety of different interleaver depths and Turbo Code rates.
The present invention is characterized, in its most basic form as a method of providing forward error correction for data services using a parallel concatenated convolutional code (PCCC) which is a Turbo Code comprising a plurality of eight-state constituent encoders wherein a plurality of data block sizes are used in conjunction with said Turbo Code.
In one variation, the method of forward error correction further uses a universal Turbo Code in a cellular mobile radio system.
In one embodiment, the method of forward error correction further uses a universal Turbo Code in a forward link and a reverse link of a cellular mobile radio system.
Specific universal Turbo Codes, with sets of optimized puncturing patterns capable of providing several commonly used code rates, are identified that provide uniformly near-optimal bit error rate and frame error rate performance over a wide range of information block sizes (hence, data rates) for a set of supported code rates.
Several universal Turbo Codes are identified herein and differ from each other in terms of: 1) the targeted code rate for which the choice of constituent encoders is optimized; and 2) flexibility with regard to the lowest code rate supported.
A suite of preferred universal Turbo Codes is provided from which a Turbo Coding forward error correction (FEC) scheme is crafted to best meet the specific design requirements of a sophisticated data communication system.